Generation of xylose solutions from Eucalyptus globulus wood by autohydrolysis-posthydrolysis processes: posthydrolysis kinetics.

Eucalyptus wood samples were treated with water under selected operational conditions (autohydrolysis reaction) to obtain a liquid phase containing hemicellulose-decomposition products (mainly acetylated xylooligosaccharides, xylose and acetic acid). In a further acid-catalysed step (posthydrolysis reaction), xylooligosaccharides were converted into xylose, a carbon source for further fermentation. The kinetic pattern governing the posthydrolysis step was established by reacting xylooligosaccharide-containing liquors at 100.5 degrees C, 115 degrees C, 125 degrees C or 135 degrees C in media containing 0.5, 1.0, 1.5 or 2 wt% of catalyst (sulphuric acid). The time course of the concentrations of xylooligosaccharides, xylose, furfural and acetic acid were determined, and the results were interpreted by means of a kinetic model which allowed a close reproduction of the experimental data. Almost quantitative conversion of xylooligosaccharides into xylose was achieved under a variety of experimental conditions. The first-order, kinetic coefficient for xylooligosaccharide hydrolysis (k1, h(-1)) varied with both temperature (T, K) and molar sulphuric acid concentration (C) according to the equation In k1 = 36.66 + 1.00lnC - 108.0/(8.314T). The hydrolysis of acetyl groups followed a first-order kinetics. The corresponding kinetic coefficient (ka, h(-1) was correlated with the operational conditions by the equation Inka = 26.80+ 1.18 InC - 73.37/(8.314T).